All is not well in the world’s greatest coral reef: Australia’s Great Barrier.

“I witnessed a sight underwater that no marine biologist, and no person with a love and appreciation for the natural world for that matter, wants to see,” says Jodie Rummer, a marine biologist from James Cook University, Australia.

Rummer was reporting back from the Great Barrier Reef, where she’d seen 95% coral “bleaching” in parts of the 600 mile-long northern area. “The bleaching now is not just restricted to the hard corals,” she says. “There’s also extensive bleaching in the soft corals, and it is also affecting anemones and giant clams.”

Bleaching happens when the water gets too warm (though it’s also possible when it gets too cold). The corals have a symbiotic relationship with algae living in their tissue. When the water warms, the coral becomes “stressed out” and expels the algae–and thus starts turning white.

NOAA

“This year, the combination of El Niño, climate change, and an extended period of hot summer days when the tide was exceptionally low has caused many of the corals that survived last year’s cyclone to lose their symbiotic algae and start bleaching,” Rummer says.

“It’s too early to tell precisely how many of the bleached coral will die,” said Terry Hughes, a coral reef expert at James Cook University, speaking to Australia’s ABC News. “But judging from the extreme level even the most robust corals are snow white, I’d expect to see about half of those corals die in the coming month or so.”